This invention relates generally to improved processes for the retreading of pneumatic tires and to the products resulting from such improved processes. More particularly it relates to a new and improved process for treating and using the cushion gum layer used in retreading tires and more specifically the way the cushion gum layer is used to unite the tire carcass to be retreaded to the new tread and/or to unite the ends of the new tread joined together around the tire carcass.
The retreading of pneumatic tires involves the use of three basic elements--the carcass of the tire to be retreaded, the new tread material to be applied thereto and a cushion gum layer to be placed between the carcass and the new tread to assist in adhering the one to the other.
In a conventional, prior art retreading process, the tire carcass, or casing, is prepared for retreading by first inspecting the used tire carefully to determine that no damage to the carcass took place during the earlier use, then removing sufficient old rubber from the tread area to provide a uniform surface for the application of the new tread rubber, then buffing that renewed surface to assist in the adhesion process and finally, if desired, applying a layer of adhesive to the buffed surface to provide building tack in the subsequent operations as well as to assist in the adhesion process.
The new tread is sometimes prepared by extruding unvulcanized rubber, properly mixed with known ingredients to provide a satisfactory tire tread, through a die of appropriate configuration. The new tread can be used in the unvulcanized state, or, may be first cured with an appropriate tread pattern molded into one surface. The industry identifies this as a precured tread. If precured, the tread is usually buffed and, if desired, cemented before being applied to the carcass. The buffing operation is applied not only to the surface to be united with the carcass but also to the ends of the tread which are to be joined together around the tire carcass.
Another method to provide the new tread is to extrude a strip or ribbon of unvulcanized rubber directly onto the prepared tire carcass as described in U.S. Pat. No. 4,221,253.
The cushion gum layer, also sometimes described in the prior art as the bonding layer or tie gum layer, is prepared by calendering a unvulcanized rubber composition mixed with the appropriate compounding ingredients well known in the art, to provide the necessary gauge, width and length required to be used between the carcass and the tread. The width and length of this layer is determined by the size of the tire and the gauge or thickness is usually in the range of 0.060 to 0.100 inches.
A second cushion gum layer which may be, but is not necessarily the same material as that used between the carcass and the tread, is used between the ends of the tread which are joined around the carcass.